We propose to continue our research on the crystal structures of small biologically important molecules, with an emphasis on studies that will enhance our knowledge of the nature of intermolecular forces and their relationship to molecular geometry and reactivity. Of particular interest to us are intermolecular interactions which involve hydrogen atoms with sufficient accuracy - about 0.02 - 0.03 Angstrom units - to study the effect of such interactions on the internal geometries of the molecules; these studies should lead to further insight into the nature of intermolecular forces and, in particular, should serve as groundwork in developing detailed, quantitative models for the reactions of biological systems.